Researchers study screening and treatment of new immigrants for latent stage of tuberculosis

December 05, 2002

Study findings from the Inner City Health Research Unit at St. Michael's Hospital/University of Toronto and the NewYork-Presbyterian Hospital Weill Cornell Medical Center demonstrate that screening and treating new immigrants from developing nations for the latent stage of tuberculosis infection would result in substantial public health and economic benefits. Results are published in tomorrow's issue of The New England Journal of Medicine.

Tuberculosis is one of the world's most prevalent diseases which infects nearly two billion people worldwide or roughly one-third of the world's total population, most of which live in developing nations. Tuberculosis is also one of the world's leading causes of death by an infectious disease resulting in approximately two million fatalities every year. In 1993, the World Health Organization (WHO) declared tuberculosis a "global emergency," the first disease to be given this classification in the organization's history.

When healthy individuals are exposed to tuberculosis, the body's immune system usually contains the infection, but is unable to completely eliminate it. This state is known as latent infection and even though it causes no physical illness and cannot be transmitted to others, it persists over a person's lifetime. However, latent infection has the potential to become active at any time, which usually occurs if the immune system becomes weakened. Active disease, in contrast to latent infection, is a very serious medical condition which if left untreated, can result in death as well as the spread of infection to others. Individuals identified during the latent stage of infection can be treated with antibiotics which substantially reduces their risk of developing active disease.

In industrialized nations, efforts to control tuberculosis in immigrant populations have been met with limited success. Currently, all immigrants to the United States and Canada are screened for the active stage of tuberculosis, however testing for latent infection is not part of the health screening system in either country.

In 2001, the U.S. reported that half of the nation's 16,000 cases of active tuberculosis were in foreign-born persons, with foreign-born persons making up one-tenth of the total U.S. population. In Canada, foreign-born persons account for about two-thirds of all active tuberculosis cases nationwide, and about 90 per cent of all active cases in Toronto.

"Although the bulk of the world's tuberculosis burden exists in the developing world, the phenomenon of globalization has brought it right to our doorstep, and has reminded us that we live in an increasingly interconnected world," says Dr. Kamran Khan, a specialist in infectious diseases and public health, and a clinician-scientist at St. Michael's Hospital, Inner City Health Research Unit.

"International travel and immigration are increasingly important factors that are contributing to the global spread of this disease."

Data for this study, which was conducted in the U.S., was obtained from the Centers for Disease Control and Prevention, WHO, and the U.S. Immigration and Naturalization Service. This data was used to construct a mathematical model or simulation which estimated the number of active tuberculosis cases that would be expected to develop over the lifetime of the more than 650,000 immigrants who came to the U.S. from developing nations during the year 2000. The study then assessed the health and economic implications of screening and treating those immigrants for the latent stage of tuberculosis infection.

Study results predict that in the U.S., between nine and ten thousand future cases of active tuberculosis could be averted and $60 to $90 million could be saved for each year of screening. "By preventing the development of active tuberculosis, screening could improve the health status of immigrant populations, reduce the transmission of infection to others in the community, and concurrently decrease the utilization of future health-care resources," says Khan. "Based on these findings, we strongly support a practice of screening and treating new immigrants from developing nations for latent tuberculosis infection. And while this research was done in the U.S., these results are also important for Canada and other industrialized nations around the world."

In addition to examining the health and economic implications of screening, the study also explored different treatment strategies taking into account global differences in antibiotic resistance patterns. "In some regions of the world, the bacteria that cause tuberculosis have become highly resistant to the antibiotics we use for treatment," says Khan. "This research provides health-care professionals with an important new tool that allows them to tailor the treatment of latent infection to specific antibiotic resistance patterns from around the world."

"Ultimately, controlling tuberculosis in our own backyard will depend on how successful we are at controlling it in the developing world," says Khan. "Industrialized nations must recognize that in the 21st century, tuberculosis is very much a global problem that can only be remedied with a global solution."

The Inner City Health Research Unit at St. Michael's Hospital addresses issues which are relevant across the globe and organized the first-ever international conference on Inner City Health - Improving the Health of the Disadvantaged - in October 2002.

St. Michael's Hospital is a Catholic teaching and research hospital, fully affiliated with the University of Toronto, specializing in heart and vascular disease, inner city health, trauma/neurosurgery, diabetes comprehensive care, minimal access therapeutics, and neurological and musculoskeletal disorders.
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The Department of Public Health of NewYork-Presbyterian Weill Cornell and Weill Cornell Medical College in New York generously supported the research of Dr. Khan, while he was a resident/fellow in preventive medicine in the Department of Public Health at Weill Cornell.

University of Toronto

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